Adoptive immunization experiments will be performed to determine the subclass identity and properties of the T-cells that mediate active T-cell-mediated anti-Listeria immunity, and those that carry immunologic memory and initiate delayed sensitivity reactions. These T-cells will be characterized in terms of their surface Lyt and Ia antigens, their anatomical distribution, migratory propensities, and life spans. A second study will investigate, with appropriate radiolabelling techniques, whether the generation of the mediator T-cells of anti-microbial immunity is concordant with increased production of blood monocytes, as measured by an increased capacity of the host during infection to direct monocytes into peritoneal inflammatory exudates. A parallel study will determine whether an increased infection-induced capacity to mobilize blood monocytes is associated with an enhanced capacity of these cells to destroy bacteria in vivo and in vitro.